The present invention relates to linear motors and, more particularly, to DC linear motors for controlling the position of an object along an axis.
Applications in which a DC linear motor may be employed include the driving of a positioning table along an axis.
Positioning tables are commonly used for moving a work object such as, for example, an electronic device in a precise path for performing an operation or inspection on the work object. Desirable characteristics of such positioning tables include precision, compactness, the maximum speed at which the table can be driven and the accuracy with which the table may be positioned.
Conventionally, positioning tables have been driven by a motor affixed at one end of a frame rotating a lead screw which extends in the direction of motion of the table and is supported at its outboard end by a precision bearing. A recirculating ball nut affixed to the table engages the lead screw whereby rotation of the lead screw urges the recirculating ball nut and the attached table to move with respect to the frame.
Certain problems are inherent in lead screw drive. Precision lead screws and recirculating ball nuts are expensive and, with the rotor of the drive motor, such components present a substantial inertia resisting the desired rapid accelerations and decelerations necessary to achieve agility in positioning the table. Furthermore, a certain amount of windup in the screw and ball nut arrangement is unavoidable. In addition to the above, great precision is required to aligning the lead screw with the direction of displacement of the table. If proper alignment is not achieved, positioning accuracy is seriously affected. Since the maximum linear speed of the table is limited by the speed at which the lead screw can be rotated, it is difficult to achieve high linear speed combined with accurate positioning.
Typically, screw driven tables of the prior art have included drive motors mounted at one end of the frame and extending outward from the frame. Thus, a greater space must be provided for the larger planform of the assembly including the additional length of the extending motor.
U.S. Pat. No. 4,151,447 discloses a linear DC motor having rows of pairs of vertically standing permanent magnets between which flat coils are arranged to travel. The polarity of DC power to the flat coils is switched by a magnetic field or electrooptical sensor at predetermined points in the travel of the flat coils. The apparatus in this patent employs trailing cables for feeding power to the coils.